Rare earth doped single polarization double clad optical fiber with plurality of air holes

ABSTRACT

An optical fiber including: (i) a silica based, rare earth doped core having a first index of refraction n 1 ; (ii) a silica based inner cladding surrounding the core and having a second index of refraction n 2 , such that n 1 &gt;n 2 , said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber; (iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n 3 , such that n 2 &gt;n 3 , wherein said optical fiber supports a single polarization mode within the operating wavelength range.

Parts of this invention were made with Government support underAgreement No. MDA972-02-3-004 awarded by DARPA. The Government may havecertain rights in some of the claims of the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical waveguide fibers, andmore particularly to a rare earth doped optical fiber exhibiting singlepolarization properties.

2. Technical Background

FIELD OF THE INVENTION

The present invention relates generally to double clad rare earth dopedoptical fibers, and particularly to all glass rare earth doped opticalfibers suitable for use with high power light sources or in opticalfiber lasers and optical amplifiers.

TECHNICAL BACKGROUND

Optical fiber has become a favorite medium for telecommunications due toits high capacity and immunity to electrical noise. Single clad rareearth doped optical fiber has been widely used in the field of opticalamplifiers and fiber lasers. This type of fiber has low capability ofhandling high power multimode optical sources due to the difficulty ofefficiently coupling multimode light from a high power optical (light)source (also referred to herein as optical pump or pump) into therare-earth doped fiber core.

To solve this problem and to increase the output power of fiber lasers,those of skill in the art utilize optical fiber with a double cladstructure (referred herein as double clad optical fiber). Double cladrare-earth doped optical fiber is a fiber that has a core, an innercladding layer surrounding the core and an outer cladding layersurrounding the inner cladding layer. Optical fibers with Yb doped coresand two cladding layers surrounding the core are disclosed, for example,in U.S. Pat. Nos. 6,477,307; 6,483,973; 5,966,491 and 5,949,941.

Double clad optical fiber has been used in applications requiringutilization of optical sources providing between 10 to 100 Watts ofoptical power, because double clad optical fiber is more efficient inretaining/utilizing optical power provided by the pump than single cladoptical fiber. This higher efficiency is due to fiber's utilization ofclad-to-core coupling of optical pump power. More specifically,rare-earth doped double clad optical fibers accept light from theoptical pump into the inner cladding and then transfer light to therare-earth doped core through the core-to-inner cladding interface,along the length of the optical fiber. Thus, the optical fiber convertsa significant part of the multi-mode light propagated through the innercladding into a single-mode output at a longer wavelength, by couplingthis pump light into the rare-earth doped core.

he inner cladding of the double clad optical fiber has a higher index ofrefraction than the outer cladding, thus the pump energy is confinedinside the inner cladding and is re-directed into the core. The opticalfiber is optically active due to the presence of rare-earth dopant inthe core, which can be excited to higher electronic energy levels whenthe optical fiber is pumped by a strong optical pump. Cladding pumpingcan be utilized in fiber amplifiers, or employed to build high-powersingle mode fiber pump lasers.

The single-stripe broad-area diode laser remains the most efficient andleast expensive pump source. Recent progress in semiconductor lasertechnology has led to creation of a single-stripe multi mode broad-arealaser diodes with output powers of more than 10 Watts.

Recent progress in semiconductor laser technology has led to thecreation of light sources utilizing either single stripe broad-arealaser diodes or laser diode bars, directly coupled to the intermediatefiber incorporated within the light source. The maximum output power ofthese light sources is more than 150 Watt at a wavelength of 976 nm atthe output end of the intermediate fiber. The intermediate fiberdiameter and numerical aperture NA of the light source is 200 μm and0.22, respectively.

In a double-clad laser, an outer cladding of the optical fiber confinesthe pump light provided by an optical pump in the optical fiber'smulti-mode inner cladding. The much smaller cross-sectional area of theoptical fiber's core is typically doped with at least one rare-earthelement, for example, neodymium or ytterbium, to provide lasingcapability in a single-mode output signal. Typically, a neodymium- orytterbium-doped double-clad fiber is pumped with one or severalhigh-power broad-area diode lasers (at 800 nm or 915 nm) to produce asingle transverse mode output (at the neodymium four-level transition of1060 nm or the ytterbium four level transition of 1030 nm-1120 nm,respectively). Thus, conventional double-clad arrangements facilitatepumping of the fiber using a multi-mode inert cladding for accepting andtransferring pump energy to a core along the length of the device.Double-clad laser output can also be used to pump a cascaded Raman laserto convert the wavelength to around 1480 nm, which is suitable forpumping erbium.

How much pump light can be coupled into a double-clad fiber's innercladding depends on the cladding size and numerical aperture NA. As isknown, the “etendue” (numerical aperture multiplied by the aperturedimension or spot size) of the inner cladding should be equal to orgreater than the etendue of the optical pump for efficient coupling. Ifthe numerical aperture and spot size of the optical source (optical pumpare) be different in both axes, in order to have better couplingefficiency, the etendue of the inner cladding should be maintained orexceed that of the pump in both the x and y directions.

Typically, a high numerical aperture NA of the inner cladding, which isrelated to the difference in refractive index between the inner andouter cladding, is desired. In the well-known design, the first cladlayer (inner cladding) is made of glass and the second layer (outercladding) is made of plastic (for example, fluorinated polymer) withrelatively low refractive index in order to increase the numericalaperture NA of the inner cladding. Such plastic may not have the desiredthermal stability for many applications, may delaminate from the firstcladding, and may be susceptible to moisture damage. In addition, thistype of double clad optical fiber may be suitable only for sustained usewith relatively low power (lower than 20 Watts) optical sources. Whenhigh power sources (more than 100 Watts) are utilized, this type ofoptical fiber heats and the polymer material of the outer cladding layercarbonizes or burns, resulting in device failure, especially when thefiber is bent. At medium powers (20 Watts to below 100 Watts), thepolymer outer cladding ages relatively quickly, losing its mechanicaland optical characteristics and becoming brittle, thus shortening thedevice life.

All-glass, Yb doped optical fibers are also known. An example of suchfiber is disclosed in U.S. Pat. No. 6,411,762. The disclosed fiber,however, is not suitable for high power applications because it has arelatively low outer cladding diameter and NA, and therefore, lowcoupling efficiency due to light leakage outside of the optical fiber.That is, a relatively large portion of the light does not enter theoptical fiber and is lost. Although this may not be an issue inapplications when only a small amount of optical power needs to becoupled into the fiber, such fiber is not efficient for high powerapplications when the light source power is 100 Watts or more.

Single polarization optical fibers are useful for ultra-high speedtransmission systems or for use as a coupler fiber for use with, andconnection to, optical components (lasers, EDFAs, optical instruments,interferometric sensors, gyroscopes, etc.). The polarizationcharacteristic (single polarization) propagates one, and only one, oftwo orthogonally polarized polarizations within a single polarizationband while suppressing the other polarization by dramatically increasingits transmission loss.

Polarization retaining fibers (sometimes referred to as a polarizationmaintaining fibers) can maintain the input polarizations on twogenerally-orthogonal axes. These fibers are not single polarizationfibers. A common polarization maintaining fiber includes stressbirefringence members and includes, as shown in FIG. 1A, a circular core12′ surrounded by an cladding region 14′. Core 12′ and the claddingregion 14′ are formed of conventional materials employed in theformation of optical waveguides. The refractive index of the corematerial is greater than that of the cladding material.

In FIG. 1A, diametrically opposed relative to core 12′, are twostress-inducing regions 13′ formed of a glass material having a ThermalCoefficient of Expansion (TCE) different from that of cladding material14′. When such a fiber is drawn, the longitudinally-extendingstress-inducing regions 13′ and the cladding region will shrinkdifferent amounts, whereby regions 13′ will be put into a state oftension or compression strain. Strain induced birefringence (otherwisereferred to a stress-induced birefringence) is imparted in the fiber andthereby reduces coupling between the two orthogonally polarizedfundamental modes. It should be recognized that such fibers includingthese stress-inducing regions 13′ do not provide single polarizationproperties.

Slight improvement in the polarization performance of single modeoptical fibers has been achieved by elongating or distorting the fibercore geometry, as a means of decoupling the differently polarized lightcomponents. Examples of such optical fiber waveguides with elongatedcores are disclosed in U.S. Pat. Nos. 4,184,859, 4,274,854 and4,307,938. However, the noncircular geometry of the core alone is,generally, not sufficient to provide the desired single polarizationproperties. It is also noted that this type of optical fiber hasrelatively low birefringence (i.e., 10⁻⁵ or less). Furthermore, thesefibers are not optically active fibers and, therefore are not suitablefor use as a laser or an amplifier fiber.

It has, therefore, been an area of ongoing development to obtain anoptical fiber that will single polarization performance while beingsuitable for use as optical amplification medium, and which is alsoeasily manufacturable.

SUMMARY OF THE INVENTION

Definitions:

The following definitions and terminology are commonly used in the art.

Refractive index profile—the refractive index profile is therelationship between the refractive index (Δ %) and the optical fiberradius (as measured from the centerline of the optical fiber) over aselected portion of the fiber.

Birefringence—birefringence is the difference between the effectiverefractive indices of the two polarization modes.

Radii—the radii of the segments of the fiber are generally defined interms of points where the index of refraction of the material used takeson a different composition. For example, the central core has an innerradius of zero because the first point of the segment is on thecenterline. The outer radius of the central core segment is the radiusdrawn from the waveguide centerline to the last point of the refractiveindex of the central core having a positive delta. For a segment havinga first point away from the centerline, the radius of the waveguidecenterline to the location of its first refractive index point is theinner radius of that segment. Likewise, the radius from the waveguide tocenterline to the location of the last refractive index point of thesegment is the outer radius of that segment. For example, an down-dopedannular segment surrounding the central core would have an outer radiilocated at the interface between the annular segment and the cladding.

Relative refractive index percent Δ % —the term Δ % represents arelative measure of refractive index defined by the equation:Δ %=100×(n _(i) ² −n _(c) ²)/2n _(i) ²where Δ % is the maximum refractive index of the index profile segmentdenoted as i, and n_(c), the reference refractive index, is taken to bethe refractive index of the cladding layer. Every point in the segmenthas an associated relative index measured relative to the cladding.

In accordance with some embodiments of the present invention, an opticalfiber is provided which exhibits polarization maintaining (retaining)properties while being suitable for use as an optical amplificationmedia. In accordance with some of the embodiments of the presentinvention, a rare earth doped optical fiber is provided which exhibitssingle polarization properties within a Single Polarization Band (SPB).The fibers parameters are preferably selected such that the SPBcoincides with an operating wavelength band.

According to the present invention the optical fiber includes:

-   -   (i) a silica based, rare earth doped core having a first index        of refraction n₁;    -   (ii) a silica based inner cladding surrounding the core and        having a second index of refraction n₂, such that n₁>n₂, the        inner cladding having a plurality of air holes extending        longitudinally through the length of said optical fiber;    -   (iii) a silica based outer cladding surrounding the inner        cladding and having a third index of refraction n₃, such that        n₂>n_(3;)        wherein the optical fiber supports a single polarization mode        within the operating wavelength range.

One advantage of the optical fiber of the present invention is itscapability to produce gain, thus being capable for use in a laser or anoptical amplifier while (i) performing as single polarization fiber andexhibiting a single polarization band SPB width of greater than 10 nmand even more preferably greater than 15 nm, and (ii) being capable ofhandling relatively large amounts of optical power. Another advantage ofthe optical fiber of the present invention is that because it performsboth as a gain fiber and the SP fiber, it eliminates the need to forsplicing together gain fiber and the single polarization fiber, therebyreducing the splicing loss, the overall fiber length, while eliminatingwork and cost associated with splicing the two fibers together.

More particularly it is believed that in these embodiments the effectiverefractive index of one of the polarizations is such that thispolarization cannot propagate within the SPB, while the other orthogonalpolarization associated with different effective refractive index issuch that this polarization may still propagate in the SPB. Accordingly,single polarization propagation within the SPB is provided by the rareearth doped fiber with a relative simple structure. In some of theembodiments of the optical fibers according to the present invention theSPB width is 20 to 40 nm.

Additional features and advantages of the invention will be set forth inthe detail description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view of a prior art opticalfiber;

FIG. 1B is a schematic cross-sectional view of one embodiment of thepresent invention;

FIGS. 2A-2C are schematic cross-sectional views of other embodiments ofthe present invention;

FIGS. 3A and 3B illustrate schematically relative a refractive indexprofiles of an exemplary optical fiber of the present invention, acrosstwo different cross-sections of the fiber;

FIG. 3C is a measured refractive index profile of a first example ofoptical fiber in accordance with the present invention;

FIG. 4 is a graph illustrating passive core loss vs. wavelength of theoptical fiber of FIG. 2B;

FIG. 5A is a graph of output power vs. launched power for the opticalfiber of FIG. 2B;

FIG. 5B is a graph illustrating single polarization property of theoptical fiber of FIG. 2B;

FIG. 6 is a refractive index profile of an alternative example of theoptical fiber of the present invention;

FIG. 7 is a graph illustrating passive core loss vs. wavelength of theoptical fiber of FIG. 6;

FIG. 8 is a schematic illustration of AlCl₃ delivery mechanism;

FIG. 9 illustrates Al₂O₃ concentration in a preform which resulted fromArgon gas delivery (bottom curve) and heated Helium gas delivery (topcurve);

FIG. 10 is a graph illustrating Yb₂O₃ and Al₂O₃ concentration within acore optical fiber preform;

FIG. 11 is a schematic illustration of the formation of a core sootpreform;

FIG. 12 illustrates consolidation of a soot preform into a glasspreform;

FIG. 13 illustrates inner cladding background loss of an exemplaryfiber;

FIG. 14 illustrates schematically a core cane utilized to manufacturethe fiber of FIGS. 1B and 2A-2C;

FIG. 15 illustrates schematically a core-clad cane utilized tomanufacture the fiber of FIGS. 1B and 2A-2C;

FIG. 16 illustrates schematically a grooved cane utilized to manufacturethe fiber of FIGS. 1B and 2A-2C;

FIG. 17 illustrates schematically a glass tube with the inserted groovedcane of FIG. 16;

FIG. 18 illustrates schematically an exemplary consolidation processutilized to manufacture the fiber of FIGS. 1 and 2A-2C;

FIG. 19 illustrates schematically a redraw tower utilized to manufacturethe fiber of FIGS. 1B and 2A-2C;

FIG. 20 illustrates a preform subassembly that includes a silica tubeoverclad with silica soot;

FIG. 21 illustrates a machined core/inner clad blank;

FIG. 22 illustrates a consolidated blank after the core/inner claddingblank it has been machined as shown in FIG. 21 and overclad with thesilica based outer cladding material;

FIG. 23 illustrates schematically a process for drawing fiber utilizedto manufacture the fiber of FIGS. 1B and 2A-2C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE SINGLEPOLARIZATION DOUBLE CLAD OPTICAL FIBER WITH AIR HOLES

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Whenever possible, the same reference numeralswill be used throughout the drawings to refer to the same or like parts.One embodiment of double clad single polarization optical fiber inaccordance with the present invention is shown schematically in FIG. 1B,and is designated generally throughout by the reference numeral 10. Theoptical fiber 10 illustrated in FIG. 1B includes: an elongated, silicabased, rare earth doped core 12 having a first index of refraction n₁; afirst silica based cladding 14 (inner cladding) surrounding the core 12and having a second index of refraction n₂, such that n₁>n₂; and asilica based outer cladding 16 surrounding the first cladding 14 andhaving a third index of refraction n₃. The inner cladding 14 contains atleast two air holes 24, 26, preferably situated on diametricallyopposite sides of the core 12 and extending along the core 12, throughthe length of the fiber 10. The air holes 24, 26 lower the effectiverefractive index of the inner cladding 14 along the line A-A illustratedin FIG. 1B and enhance or enable single polarization property of thisfiber. The core 12, inner cladding 14 and the outer cladding 16 are madeof glass. A protective coating 18 surrounds the outer cladding 16. Theouter coating 18 may be, for example, an organic coating which typicallyincludes a softer primary coating and a harder secondary coating appliedover the primary coating.

In this embodiment the silica based core 12 is doped with Yb, but otherrare earth materials, such as Er may also be utilized. The core 12 mayalso include at least one index raising dopant. The outer claddingfurther 16 preferably includes an index lowering dopant, such that n₂>n₃The inner cladding diameter D_(IN) is preferably at least 125 μm andmore preferably at least 200 μm. It is even more preferable that innercladding diameter D_(IN) is at least 225 μm and most preferable at least250 μm. Applicants discovered that the thick inner cladding 14 andall-glass construction of the optical fiber work in synergy to allow theoptical fiber to be coupled to high energy source, and to couple thehigh power into the core without damaging the optical fiber, while twoair holes make this fiber a single polarization fiber. The size of theair holes may vary, preferably from 7 to 20 μm in diameter, depending onthe desired size (minor axis)of the fiber core.

It is preferable that the outer cladding 16 be relatively thin, withwall thickness less than 80 μm and preferably between about 5 μm and 35μm. It is most preferable that the wall thickness of the outer cladding16 be between about 10 μm to 25 μm. It is preferable that the diameterD_(C) of the fiber core 12 be about 5 μm to 20 μm, the inner claddingdiameter D_(IN) be about 125 μm to 2000 μm and more preferably about 125μm to 1500 μm. It is even more preferable that D_(IN) be about 125 μm to350 μm. It is preferable that the diameter of the outer claddingdiameter (D_(OUT)) be about 145 to 2100 μm, more preferably betweenabout 145 μm to 1600 μm and even more preferable that D_(OUT) be about145 μm to 500 μm. If the inner cladding 14 does not have a circularcross section, Din is defined as the smallest distance from one side ofthe inner cladding's cross section to the oppositely situated side ofthe cross section. It is also noted that the outer cladding 16 may notbe circular. If the outer cladding 16 is not circular, D_(OUT) isdefined as the smallest distance from one side of the outer cladding'scross section to the oppositely situated side of the outer cladding'scross section. It is preferable that the inner cladding's 14cross-sectional area be at least 200 times larger than the crosssectional area of the core 12. It is even more preferable that the crosssectional area of the inner cladding 14 be between 300 and 3000 timeslarger than the cross sectional area of the core 12. For example, thecross sectional area of the inner cladding 16 may be 500, 700, 1000,1200, 1500, 1600, 2000 or 2500 times larger than the cross sectionalarea of the core 12.

According to this embodiment, the fiber core 12 includes, in weightpercent: Rare earth 0.1 to 2.5 wt %; P 0 to 5 wt %; Al 0.5 to 15 wt %;Ge 0.1 to 15 wt %; F 0 to 1 wt %.

The rare earth dopants in the fiber core 12 provide active ions toenable either a gain or a lasing action. Exemplary rare earth dopantsare Yb, Er, Nd, Tm, Sm and Tb. It is preferable that the amount of rareearth dopant in the core 12 be 0.5 wt % to 1.5 wt %. Phosphorus may beadded to the core materials in order to lower the softening temperatureof the core glass, which may be advantageous if the core is produced bythe inside vapor deposition process. Phosphorus may also be utilized asa refractive index raising agent. However too much phosphorus (10% ormore) provides nonlinearity through Stimulated Raman Scattering whichmay inhibit the lasing action. Aluminum may be added to the core as ade-clustering agent (for example, to de-cluster Yb, preferably at theratio of Al to Yb of 5:1 to 10:1). The core 12 may also includeGermanium which is an index raising dopant, and/or fluorine which is anindex lowering dopant as well as a de-clustering agent.

The preferred ranges of the core 12 composition in weight percent are:Rare earth 0.3 to 1 wt %; P 0 to 2 wt %; Al 2 to 8 wt %; Ge 3 to 15 wt%; and F 0.1 to 0.5 wt %.The Yb-doped core 12 will laze at 1.03-1.11 micron range.

It is preferable that the inner cladding 14 contain 5 wt % to 30 wt % Gein order to provide high NA. It is even more preferable that the innercladding comprise 5 wt % to 20 wt % Ge. It is noted that 5 wt % to 10 wt% Ge works well for many applications.

It is preferable that the index lowering dopant of the outer cladding 16comprises Fluorine and/or Boron in weight percent: F  0.5 to 5 wt %; B0.5 to 20 wt %.

The amount of dopant(s) for the outer cladding 16 is chosen topreferably result in inner cladding NA of between 0.15 to 0.5. However,it is preferable that the outer cladding 16 contain at least one of of Bor/and F. It is preferable that the amount of B is at least 3 wt %. Itis preferable to have more than 1 wt % and more preferably more than 2wt % of F along with more than 8 wt % of B in the outer cladding 16. Itis preferable that the outer cladding 16 has less than 5 wt % of F, andless than 15 wt % of B. It is even more preferable that the amount of Band F be: 2 to 4 wt % of F and 3 to 15 wt % of B.

Other embodiments of the double clad optical fiber of the presentinvention are shown schematically in FIGS. 2A-2C and are generallydescribed and depicted herein with reference to several exemplary orrepresentative embodiments with the same numbers referenced to the sameor functionally similar parts. The inner cladding 14 of the opticalfiber of FIGS. 2A-2C is non-circular. The advantage of non-circularinner cladding 14 is that non-circular shape improves the absorption ofoptical pump power into the core 12. The elongated core 12 may belocated either at the geometric center of the inner cladding, or may bedisplaced from the geometric center of the inner cladding.

The optical fiber core 12 is preferably elliptical, as shown in FIGS. 1Band 2A-2C, but may have other elongated shapes. Adjacent to the core andsituated at least partially within the inner cladding 14 are at leasttwo air holes 24, 26. The elongated (elliptical) core 12, inconjunctions with the air holes 24, 26 renders this optical fiber asingle polarization (SP) fiber. It is preferred that the aspect ratio(ratio of major to minor axis) of the elliptical core 12 be at least1.5:1 and more preferably be between 2:1 and 10:1, because these aspectratios improve birefringence of the core 12.

The core delta is less than 1% Δ and preferably less than 0.5% Δ. Thenumerical aperture NA of the core 12 is between 0.05 (for high powerlaser application) and 0.25 (for lower power application). The numericalaperture NA of the core 12 is defined as (n₁ ²-n₂ ²)^(1/2), where n₁ isthe index of refraction of the core 12 and n₂ is the index of refractionof the inner cladding 14.

The silica based inner cladding 14 may have a circular outer perimeter,as shown in FIG. 1B (preferably with an off-center situated core), or anon-circular outer perimeter as shown in FIG. 2A-C. The numericalaperture NA of the inner cladding 14 is defined as (n₂ ²-n₃ ²)^(1/2),where n₃ is the index of refraction of the outer cladding layer 16. Theinner cladding preferably has numerical aperture NA between 0.15 and0.45 and more preferably between 0.3 and 0.4.

In general, a double-clad structure that could be used in a fiber laseror in an amplifier includes two claddings. A first (inner) multi-modecladding acts as a multi-mode pumping core. The inner cladding 14 isadjacent to the core 12 and a second (outer) cladding 16 surrounds thefirst or the inner cladding 14. The core 12 may be either single mode ormulti-mode at the core lasing wavelength. The inner cladding 14 servesas a waveguide with a high numerical aperture NA for the input (pumping)light. That is, the inner cladding serves as a pump cavity. The largerthe inner cladding diameter, the more pump light is coupled into theinner cladding from the optical source. The cross-section of the firstmulti-mode inner cladding (D_(IN) is the shorter dimension of the innercladding as seen in FIGS. 2A-2C) may be designed to have a desiredshape, e.g., matched to the near field shape of the pump source or haveany other which increases coupling efficiency of the (pump) light fromthe light source to the inner cladding. The numerical aperture of theinner cladding must be high enough to capture the output of the lightsource, such as the laser diode. Recent progress in semiconductor lasertechnology has led to the creation of light sources utilizing discreteor arrayed broad-area laser diodes coupled to the intermediate fiberincorporated within the light source. The output power of this lightsource is more than 150 Watt at 976 nm at the output end of theintermediate fiber. The diameter of the intermediate fiber and NA oflight source is 200 μm and 0.22 NA, respectively.

The light from this light source is then coupled to a double cladoptical fiber via high NA and large aperture lenses. With this approachone can obtain 85-90% of coupling efficiency.

EXAMPLES

The invention will be further clarified by the following examples.

Example 1

FIGS. 3A and 3B illustrate schematically a relative refractive indexprofile of a first exemplary optical fiber of the present invention.More specifically, FIGS. 3A and 3B depicts optical fiber's refractiveindex percent delta (relative to that of the pure silica) vs. thedistance measured from the core center. FIG. 3A illustratesschematically a refractive index profile taken across the region thatdoes not contain the air holes, for example, along the line Y-Y of thefiber depicted in FIG. 1B. FIG. 3B illustrates schematically arefractive index profile of the same fiber, but taken across the regionthat contains the air hole 24 (for example, along the line A-A of thefiber depicted in FIG. 1B).

FIG. 3C illustrates measured refractive index profile (percent delta,relative to that of the pure silica) of a first exemplary optical fiberof the present invention along the Y-Y axis, measured from the corecenter. This optical fiber has the cross-section illustrated in FIG. 2B.The distance D_(IN) between two opposing flat sides of this innercladding cross section is 260 μm. The refractive index percent delta isdefined herein as (n_(i) ²-n_(S) ²)/2n_(i) ², where i=1, 2 or 3 andn_(S) is the refractive index of pure silica. This optical fiber has aYb doped core 12, a Ge-silica inner cladding (% delta≈0.46) and an outercladding 16 which doped with Fluorine and Boron.

FIG. 3C shows that the relative refractive index difference (percentdelta) of the core 12 is about 0.56, that the fluorine/boron doped outercladding 16 has the refractive index percent delta of about −1.4. TheYb-doped fiber core is single-mode for the wavelengths above 1 μm. Ifthe core 12 is doped with Erbium, the optical fiber will be single-modeat lasing wavelength of 1.55 μm. The optical fiber 10 has a relativelylow NA (about 0.065) for the core 12, and high NA (0.30) for the innercladding 14. (The NA is defined by (n_(i) ²-n_(i+1) ²)^(1/2).) Thisinner cladding NA is preferably higher than that of the pump-source,allowing high coupling efficiency for the pump light of 90% or better.The small core NA (0.065) enables single mode operation with a largecore size (10.5 microns diameter). If the core NA is higher (0.13, forexample), the core diameter would have to be smaller (about 5 microns,for example) in order to be single mode. The bigger core diameter andlower core NA allows the core 12 to stay single-mode, while allowing thecore to take more pump-power from the inner cladding, and also increasesfiber power handling capability. The specific composition for thisexemplary optical fiber is:

Core 12: 0.6 wt % Yb₂O₃; 4.5 wt % AL₂O₃; 3.0 wt % GeO₂; 0.2 wt % F;

Inner cladding 14: 8.5 wt % GeO₂;

Outer cladding 16: 9 wt % B and 2.7 wt % F.

The amount of each dopant is optimized to ensure the high laserefficiency. The preferred inner cladding shape is not circularlysymmetric, thus maximizing the pump absorption.

The double clad fiber produced by the OVD process is especially suitablefor use in a higher power fiber laser device. FIGS. 4 and 5 correspondto the optical fiber of FIG. 2B. More specifically, FIG. 4 illustratesthe low passive loss, for example 3 dB/km at 1280 nm, achieved in theYb-doped core of the fiber of FIG. 2B. The passive loss of the core(also referred to as a background loss) is the inherent loss from thecore materials without the absorption-effect from the active dopantssuch as Yb or Er etc. FIG. 5 illustrates the single mode fiber-laserefficiency of this fiber. More specifically, FIG. 5 is a graph of outputpower (Watts) versus input power (Watts). The optical pump wavelength is976 nm. The optical pump is fiber coupled semiconductor laser diode bars(Ga—As/InGaAs). The output from this optical pump was launched into theinner cladding 14 of the double clad optical fiber of FIG. 2B. The fiberlaser shows low threshold and lasing efficiency of 52% (which is definedby the graph's slope). The fiber has good power-handling capability andoperates well with optical sources that provide optical (pump) power ofover 100 Watts. The optical fiber 10 of this example has absorption perunit length (when launching pump power in the inner cladding) in therange of 0.1-2 dB/m.

FIG. 5B illustrates measured transmission spectrum of double clad singlepolarization fiber shown in FIG. 2B. The single polarization bandwidthis around 20 nm, centered at 1080 nm in which lasing taking place. Inthis exemplary fiber the first cutoff wavelength λ1 is about 1070 nm andthe second cutoff wavelength λ2 is about 1090 nm.

Example 2

FIG. 6 illustrates a refractive index profile of a second exemplaryoptical fiber of the present invention, across the region that does notintersect the airholes 24, 26 (along the axis Y-Y, for example). Morespecifically, FIG. 6 depicts refractive index delta as vs. the radiusfor the second exemplary optical fiber. This optical fiber has a Ybdoped, silica based core 12 which is multi mode at the lasing wavelengthof 1100 μm, a silica based inner cladding 14 having two sections ofalmost the same index of refraction (delta %≈0) and an outer cladding 16which is doped with fluorine. The NA of the inner cladding is 0.16. FIG.6 illustrates that the refractive index difference (delta %) of the core12 is about 0.7, that the fluorine doped outer cladding 16 has therefractive index delta of about −0.7.

The double clad optical fiber illustrated in FIG. 6 is also suitable foruse in a fiber laser device. FIG. 7 corresponds to the optical fiber ofFIG. 6. More specifically, FIG. 7 illustrates the low passive loss, forexample less than 2 dB/km at 1280 nm, achieved in the Yb-doped core ofthe fiber of FIG. 6. The optical fiber has good power-handlingcapability with power of over 10 Watts.

The specific composition for the optical fiber of the second example is:

Core 12: 0.8 wt % Yb₂O₃; 9.5 wt % P₂O₃; 5.4 wt % GeO₂;

Inner cladding 14: Pure Silica;

Outer cladding 16: 2.3 wt % F.

The optical fiber of example 2 would have a single polarization range of20 nm to 40 nm, depending on the size (along the minor axis) of the core12.

The Process for Making Fiber

The fiber of FIGS. 1B and 2A-2C is produced by theoutside-vapor-deposition process (OVD). The OVD process is a way ofmaking optical fiber by depositing from the desired vapor ingredients(including silica and the desired dopants) reacting with oxygen in aflame to form the soot-particles on a bait rod, for making soot-preform.The soot-preform is then consolidated into solid glass in a hightemperature furnace, after the bait rod is removed. The core/innercladding/outer cladding compositions are achieved by utilizing differentvapor-ingredients for each of the layers in the soot preform formingprocess. The core preform is generated first, then consolidated,followed by core/inner cladding preform generation and consolidation,which in turn, is followed by the outer cladding outside vapordeposition process and another consolidation step. The final preform isthen drawn into double-clad single polarization optical fiber 10 byknown fiber-drawing methods.

More specifically, the following steps are utilized to make the rareearth doped double clad single polarization fiber of FIGS. 1B and 2A-2C.

1. Core cane formation. The core cane is formed first. The core ismanufactured, for example, by a standard OVD process. The core materialsare deposited onto the bait rod during the laydown step. The exemplaryvapor-precursor-materials used to make the fiber core cane are Yb(fod)₃,AlCl₃, SiF₄, SiCl₄, GeCl₄ and tri-ethyl borate. Other rare-earthmaterials may be utilized either in addition to Yb, or instead of Yb.During the core deposition process we achieved a uniform AlCl₃ gas-phasedelivery. This was accomplished by utilizing heated inert Helium ascarrier gas 30 (instead of Argon gas) for AlCl₃ delivery illustratedschematically in FIG. 8. As solid AlCl₃ changes into vapor (gas) phase,it consumes a large amount of heat. Helium gas has high thermalconductivity; effectively transfers heat to AlCl_(3,) and maintainsconstant vapor pressure of AlCl₃. It is preferable that Helium gas isprovided at a temperature within 150° C. to 180° C. range. Asillustrated in FIG. 8, the heated Helium gas is provided by the He gasheater 32 to the oven 50 containing AlCl₃ vessel 52. The relatively highHelium gas temperature helps to maintain the AlCl₃ containing vessel 52at a constant temperature of about 140° C.-160° C. In order to make theoptical fiber of this example, Helium gas was heated via heater 32 to168° C. and the vessel 52 temperature was held constant at 145° C.Higher vessel temperature results higher concentration of Al in thepreform. In addition, the Helium gas flow rate was also adjusted for themost uniform delivery throughout the core doping process. In thisexample, a 10% flow-rate slope (liter/min) is used for the delivery.(The increase in flow rate with subsequent passes was utilized for allother dopants of the core and claddings.) Heated Helium gas carriesAlCl₃ vapor via a heated gas line 54 to the flame burner (gas burner)56. To produce the core preform of this example, a 100 passes of coredeposition process is started with 1.2 liter/min (pass #1) and ended(after pass #100) with 1.65 liter/min, resulting in soot preform corethickness of about 2 mm to 3 mm. Heated Helium based AlCl₃ delivery maybe utilized not only to form a fiber core, but to also provide Al dopingto other fiber layers (e.g. cladding), if uniform Al doping of suchlayers is desired. Furthermore, heated Helium assisted delivery may bealso utilized for materials other than AlCl₃, which are also endothermic(i.e. heat-absorbing). An Argon gas delivery instead of the Helium gasdelivery of AlCl₃ may be utilized, but a Helium gas delivery of AlCl₃results better uniformity of Al₂O₃ concentration. (See FIG. 9). It ispreferable that Al₂O₃ is evenly distributed throughout the core layerbecause its presence assists in de-clustering of rare earth dopant(s)within the core. This results-in high laser/amplifier efficiency throughreduced quenching. This delivery process can also be utilized in Aldoped (for example, in order to replace Ge) transmission fiber (i.e.fiber without rare-earth dopants in the core) when a fiber layer withrelatively high index of refraction (i.e. higher than silica) is needed.

As shown in FIG. 10, the heated Helium delivery of AlCl₃ resulted in avery uniform distribution of Yb and Al throughout the preform core,which results in uniform concentration of Yb and Al within the fibercore 12. More specifically, the resultant variability of Al₂O₃concentration in the core is less than 2 wt % and preferably less than0.5 wt % and more preferably less than 0.25 wt %, especially for maximumAl₂O₃ concentration of over 3 wt %. It is also preferable that the ratioof max wt % to min wt % of Al₂O₃ concentration in any given fiber layer(e.g. core, cladding, etc.) be less than 2:1, preferably less than1.5:1, more preferably less than 1.2:1, and even more preferably lessthan 1.1:1, especially for maximum Al₂O₃ concentration of over 3 wt %.

The Yb vapor delivery is carried by Argon gas and is accomplished byheating organometallic Yb(fod)₃ in the temperature range of 150° C.-180°C., which results in a soot preform core with Yb₂O₃ concentration fromabout 0.2 wt % to 3 wt %. In order to make the optical fiber 10 of thisexample, the Yb(fod)₃ containing vessel temperature of 163° C. was usedto achieve the Yb₂O₃ concentration of about 0.6 wt %. The delivery ofother materials is carried out by conventional oxygen delivery attemperatures below 100° C.

More specifically, according to one embodiment of the present invention,the Yb(fod)₃, AlCl₃, SiF₄, SiCl₄ and GeCl₄ are delivered to a gas burner56. (See FIG. 11.) The gas burner 56 operates at a temperature of about2000° C. The pre-determined amounts of various vapor-phase materialsdelivered for each core (or clad) stage are carried by oxygen providedto the burner 56, and react in the burner flame 58 where the desiredglass-soot particles formed. The soot particles are then deposited ontoa rotating bait-rod 59 or core cane 60 through the thermophereticmechanism to result in the designed soot-preform 62 which will be usedto manufacture single polarization fiber with the Yb-doped single-modecore.

After the core soot preform layer is layered down and the soot preform62 is cooled to room temperature, the bait rod 59 is removed from thecenter of core soot preform 62. The core soot preform 62 is thenconsolidated (densified into the solid glass) to become a solidglass-preform 62A which is drawn into core cane 62B. (See FIGS. 12 and14.)

Applicants discovered that a proper choice of high temperature and fastdown-feed rate during consolidation results in low crystallizationformation in the resulting solid glass preform, which results in anoptical fiber having very low passive (background) loss, and alsoeliminates the conventional double-redraw process associated with Aldoped blanks. More specifically, soot preform 62 is down fed relative tothe furnace at the rate and temperature sufficient to minimizecrystallization such that the background loss of the resultant fibercore is less than 8 dB/km, and preferably 3 dB or less, at a wavelengthof 1280 nm. As illustrated in FIG. 12, the ‘core’ soot preform 62 isconsolidated into solid glass-preform 62A in a high temperature (1400°C. to 1600° C.) furnace 64. It is preferred that the furnace temperatureduring consolidation be 1500° C. to 1600° C., and more preferably 1530°C. to 1580° C. In order to produce the optical fiber 10 of this examplewe utilized the furnace temperature of 1550° C. Applicants found thatfor temperatures of below 1500° C. the preform glass forms crystals andthe amount of crystallization is significantly reduced with furnacetemperatures of above 1530° C. While in the furnace, the soot preform 62is moved relative to the furnace 64 (e.g., down-fed) at a rate of 7mm/min or faster. It is preferred that this rate be 8 mm/min to 12mm/min. The optical fiber of this example made by down-feeding the sootpreform 62 at the rate of 9 mm/min. It is noted that instead ofdown-feeding the soot preform, the soot preform may be held in aconstant position and the furnace may be moved instead. Thus, byspecifying that the soot preform is moved relative to the furnace,applicants intend to cover any relative movement between the sootpreform and the furnace. Generally, it is recommended that the higherthe furnace temperature, the faster the rate of relative motion betweenthe furnace and the soot preform.

With the above described high consolidation temperatures and fastdown-feed rate, the resultant optical fiber 10 has the core backgroundloss of less than 8 dB/km. More preferably, the optical fiber exhibitscore background loss of less than 5 dB/km. In this example thebackground loss of the core is less than 3 dB/km. The core backgroundloss was measured by making (single mode) optical fiber without theouter cladding and measuring the background loss of this fiber.

The core soot preform 62 has sufficient amount of Ge to produce, afterthe cladding process is completed, a fiber with core delta of 0.06 to0.1%. After the core preform 62 has been consolidated, as describedabove, it is drawn into the core cane 62B. The core cane 62B ispreferably 1 meter long and about 8 mm in diameter. The core cane 62B isillustrated schematically in FIG. 14.

2. First clad blank formation. The core cane 62B is overclad with silicasoot to form a core/clad (soot) blank (referred herein as the first cladbank 63). The first clad blank is then consolidated to form cane 63A.The first clad blank 63 has a core to the first clad diameter ratio of0.4 to 0.6. The cane 63A is about 42 mm in diameter. Cane 63A isillustrated schematically in FIG. 15.

Alternatively a sleeving process may be utilized to form cane 63A, byplacing a silica sleeve around the core cane 62A.

3. Grooved cane formation. The cane 63A includes sections 112, 114 whichcorrespond to the core 12 and the first cladding layer 14 of the opticalfiber 10. Cane 63A is preferably about 1 meter long and about 8 mm indiameter. Grooves 54 are then ground into the diametrically oppositelongitudinal sides of the cane 63A to a width of about 6.4 mm and to adepth of about 8 to 10 mm, thereby forming grooved cane 63B. (See FIG.13.) The groove depth depends on the thickness of the first clad layer,but should be such that its bottom substantially abuts the section 112(corresponding to the fiber core 12), as illustrated in FIG. 18. Thegrooved cane 63B is HF etched for about 30 minutes to clean any grindingresidue and then redrawn to an appropriate size cane (OD of about 8 mm).

The grooved and redrawn cane 63B is then inserted into a 1 meter longsilica tube or sleeve 65 overclad with silica soot 67 (for example,about 800-1000 gms.), as shown in FIG. 19, to form a preform subassembly70. Silica overcladding 67 method on the sleeve 65 is preferablyproduced by an Outside Vapor Deposition (OVD). The exemplary silica tube65 may have an inner diameter of about 8.8 mm and an outer diameter ofabout 11.8 mm which supports a layer of silica soot 67. The silica tubebaring soot is cleaned, both inside and outside, with a chemical solventor alcohol (IPA for example), prior to the insertion of the etched andre-drawn cane 63B into the tube 65. If needed, the two holes 24, 26 inthe preform subassembly 70 may be further etched via HF to enlarge theholes.

The preform subassembly 70 of FIG. 17 is then consolidated in accordancewith a conventional consolidation process as shown in FIG. 18 by firstdrying in a consolidation furnace 64 in an atmosphere of Cl₂, and thenconsolidating in the furnace in a He-containing atmosphere to produce aconsolidated preform 70A. The consolidated preform 70A is then insertedinto a redraw tower 74 as shown in FIG. 19. The preferred down feedingrate is about 7 mm/min. Heat is applied to preform 70A by heatingelement 75 and it is drawn down by tension applying wheels 76 into anapproximately 7-8 mm diameter cane 78. While the redraw process (drawingto a smaller diameter core cane from the preform) is occurring, apositive pressure (about 1 psi) is applied to the holes 24, 26sufficient to keep them from closing. The pressure may be sufficient tocause the central core to elongate slightly. The pressure used is afunction of the draw temperature, glass viscosity, and draw speed amongother factors.

This cane 78, now having an elliptically shaped central core and airholes, is again inserted into a 1 meter long silica tube 65A which isoverclad with about 1000 grams of silica soot 67A, as shown in FIG. 20to form preform subassembly 70B. This preform subassembly 70B isconsolidated in the same manner as heretofore to form consolidatedblanks 70C. The consolidated blanks 70C will form the basis for the coreand the inner clad portion of the optical fiber 10.

The consolidated blanks 70C are then machined, if needed, to desiredshape. Breaking circular symmetry in the inner clad layer enhances pumplight absorption efficiency. A machined core/inner cladding blank 70D isillustrated schematically in FIG. 21. The machined blank 70D is overcladagain, for example by SiO₂ with index lowering dopants and thenconsolidated to a consolidated blank 71. The down-doped silica layer ofthe consolidated blank 71 will form the second, or outer cladding 16 ofthe optical fiber 10. FIG. 22 illustrates schematically an exemplaryconsolidated blank 71. If boron is used in overcladding, it is preferredthat consolidation is performed in Fluorine environment. In thisexample, the index lowering dopants are B and F.

More specifically, B₂O₃ and SiO₂ were vapor deposited on the groundglass preform to form a B₂O₃ and SiO₂ soot layer by using tri-ethylborate and SiCl₄ delivered to the burner. The blank (i.e. machined orground glass preform) covered with the B₂O₃—doped silica soot layer wasthen Fluorine doped during the consolidation step by using SiF₄ gasprovided to the consolidation furnace. During this second consolidationstep, the consolidation furnace is operated at the temperature range of1300° C.-1400° C. At these consolidation temperatures Fluorine diffusesinto the boron/silica soot layer, but does not penetrate into theunderlying glass layer. The optical fiber of this example was producedby utilizing consolidation temperature of 1350° C., so as to facilitateadequate Fluorine doping through diffusion. In this example, the thirdlayer of the preform (outer cladding) has a shape similar to that of thesecond layer (inner cladding).

The consolidated blank 71 is then suspended from a handle 81 in a drawfurnace 80 as shown in FIG. 23 and a fiber 82 is drawn therefrom. Duringdraw, a small positive pressure (about 1 psi or less) is applied to theholes to keep them from closing. This causes the core to become (more)elliptically shaped. In the exemplary fiber depicted in FIGS. 1B and2A-2C we utilized positive pressure of less than 0.1 psi. The draw speedwas about 1 m/sec. The resulting fiber has an elliptically shaped coreand dual air holes.

As should be recognized, the elongation of the core may occur in theredraw step, the draw step, or combinations thereof to achieve thedesired aspect ratio of the central core. In either case, a positivepressure is applied to the holes in the preform (and fiber) to cause theelongation to occur.

It will be apparent to those skilled in the art that variations andmodifications can be made to the present invention without departingfrom the scope of the invention. For example, although step indexstructures are show, other graded index structures may be employed.Moreover a ring structure may be added to the fiber profile as well andwould still function acceptably. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1. An optical fiber comprising: (iv). a silica based, rare earth dopedcore having a first index of refraction n₁; (v). a silica based innercladding surrounding the core and having a second index of refractionn₂, such that n₁>n₂, said inner cladding having a plurality of air holesextending longitudinally through the length of said optical fiber; (vi)a silica based outer cladding surrounding said inner cladding and havinga third index of refraction n₃, such that n₂>n₃; wherein said opticalfiber supports a single polarization mode within the operatingwavelength range.
 2. The optical fiber according to claim 1 wherein saidrare earth core is an elongated core.
 3. The optical fiber according toclaim 1 wherein said core is elliptical and the ratio of its maximumdimension to its minimum dimension is at least 1:1.5.
 4. The opticalfiber according to claim 1, wherein said core is doped with Al₂O₃, suchthat the ratio of max wt % to min wt % of Al₂O₃ concentration in saidcore is less than 2:1.
 5. The optical fiber according to claim 1 whereinsaid rare earth core includes Yb.
 6. The optical fiber according toclaim 1 wherein said air holes are circular with a diameter of 7 to 20μm.
 7. The optical fiber according to claim 1 wherein said innercladding contains an even number of air holes.
 8. The optical fiberaccording to claim 1 wherein the shortest dimension of said innercladding is at least 145 μm.
 9. The optical fiber according to claim 1wherein said silica based outer cladding contains at least F or Boron.10. The optical fiber according to claim 1 wherein the core delta is0.5% Δ or less.
 11. The optical fiber according to claim 2 wherein thecore has a a maximum dimension of 1.5 μm to 15 μm and a minimumdimension of 1 μm to 5 μm.
 12. The optical fiber according to claim 1wherein said operating wavelength range is 1000 to 1120 nm.
 13. A methodof manufacturing optical fiber comprising the steps of: (i) providing apreform having a rare earth doped core overclad with a silica basedlayer; (ii) providing a plurality of longitudinal groves within saidsilica based layer, thereby making a grooved preform, (iii) insertingsaid grooved preform into silica based tube, thereby forming a pluralityof longitudinal holes within said tube; (iv) drawing the optical fiberwhile applying positive pressure to said holes.
 14. A method ofmanufacturing optical fiber comprising the steps of: (i) providing apreform having a rare earth doped core overclad with a silica basedlayer; (ii) providing a plurality of longitudinal groves within saidsilica based layer, thereby making a grooved preform, (iii) insertingsaid grooved preform into silica based tube overclad with silica soot;(iv) consolidating said soot while applying positive pressure to thegrooved areas, thereby forming a glass preform containing a plurality oflongitudinal holes; (v) utilizing said glass preform containing aplurality of longitudinal holes by surrounding it with a layer of downdoped silica soot; (vi) consolidation said glass preform containingdowndoped silica soot to form an all glass preform, while retaining saidplurality of air holes; (vii) drawing the optical fiber while applyingpositive pressure to said holes.
 15. The method of manufacturing opticalfiber according to claim 14, said method further comprising the step of:machining said forming a glass preform containing said plurality oflongitudinal holes to a non-circular shape, prior to surrounding it witha layer of down doped silica soot.